When Mutant/+ animals were intercrossed, progeny that were homozygous for the mutagenized chromosome across the 95% confidence interval did not survive beyond postnatal day one (P1), despite the fact that homozygous mutants were phenotypically indistinguishable from wild-type pups at birth and were born at the expected Mendelian ratios. We hypothesized that the same mutation was responsible for the locomotor, weight and lethal phenotypes and used the lethal phenotype to fine map the mutation. We intercrossed mutant carriers extensively and genotyped animals that survived to adulthood. Haplotype analysis indicated that lethality mapped to a small region between the markers D12Mit180 and D12Mit195 (Figure 3A, left panel). We confirmed this observation by crossing three different males with recombination events surrounding these markers to Mutant/+ females. Analysis of survivors indicated that lethality was indeed caused by a mutation within this region (Figure 3A, right panel). Additional single nucleotide polymorphisms narrowed the non-recombinant interval to ∼1.5 Mb. Exon sequencing of seven candidate genes within this region revealed a single nonsense point mutation in the mouse homolog of the C. elegans unc-79 gene. The mutation
